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BMW i3 has entry-level electric prowess

July 28, 2011

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While the overtly sporting i8 is set to act as the performance figurehead for BMW's initial range of purpose-built alternative-drive vehicles, it is the less expensive and the more practical i3 that offers the choice of either standard plug-in electric or optional range-extended electric drive. This feature will provide BMW's “i” brand with broad appeal at a base price that executives suggest will be comparable to more powerful versions of today's 1-series when sales begin in 2013.

Conceived primarily for urban driving, the compact zero-emissions hatchback eschews the traditional low-slung lines of contemporary BMW small cars for a distinctly more upright stance that offers the i3 excellent all-round visibility for ease of maneuverability in city traffic, along with room for up to four adults and a nominal 7.1 cubic feet of luggage space behind the rear seats. In BMW's words, “It's been designed for maximum practicality but without any compromise in overall driving dynamics.”

As with the i8, the i3 is based around a light but highly resilient carbon-fiber-reinforced plastic body structure and rides on a predominately aluminum chassis in a move that will see the car hit the scales in production trim at about 2,800 pounds.

At 151.4 inches in length, 79.2 inches in width and 60.5 inches in height, the five-door i3 is 4.7 inches longer, 12.8 inches wider and 5.2 inches higher than the three-door Mini hatchback. It also rides on a platform boasting a 4.1-inch-longer wheelbase, at 101.2 inches, and wider tracks.

BMW claims the rear-wheel-drive i3 boasts a “comparatively small turning circle” and keenly describes its handling as being “agile at typical city speeds.” By mounting the car's liquid-cooled battery--a lithium-ion unit of unspecified weight and capacity--within the central section of the floor, BMW says it achieved a center of gravity that is lower than that of existing gasoline- and diesel-powered 1-series models.

Unlike the gasoline-electric-hybrid-powered i8, the i3 relies on a pure electric driveline currently undergoing durability testing in a fleet of 1-series Active-E prototypes. At the heart of the new plug-in system is a compact electric motor mounted over the rear axle, where it provides drive to the rear wheels via a single-speed transmission.

The rear mounting of the engine has been made possible by a continuous development program kicked off in 2009 that has seen the size of the electric motor reduced by 40 percent compared with that used in the Mini E, a car that has acted as a test bed for much of the i3's driveline hardware.

BMW says the drive unit--motor, power electrics, transmission and differential--are mounted so that they do not intrude into the interior.

With 170 hp and 184 lb-ft of torque, the brushless motor is claimed to propel the i3 from to 0 to 62 mph in 7.9 seconds. It is also claimed to possess 50-to-75-mph rolling acceleration of 6.0 seconds. BMW has yet to provide details about the i3's aerodynamic properties, but without the need for drag-inducing cooling ducts and the inclusion of skinny, low-rolling-resistance tires, its drag co-efficient is expected to be less than 0.30.

Maximum speed is limited to 93 mph to protect the charge of the battery. “We could gear it to achieve a much higher top speed, but in the interests of range, we have decided to cap it at a level we think will be sufficient for most needs,” a BMW insider told AutoWeek. The i3's battery has undergone detailed optimization to allow it to operate in all climates.

As with the Mini E, BMW say it will give its urban-based electric car a heavy acting brake-energy-recuperation system with prodigious regeneration of electricity on a trailing throttle. It will also get a so-called coast-mode function that can be chosen by the driver for periods of energy-neutral freewheeling. While it is holding back on exact details of the i3's lithium-ion battery pack until closer to launch, BMW claims an overall range of between 80 and 100 miles for the i3.

Internal studies reveal that the i3 is sufficient for up to 90 percent of journeys likely to be undertaken by customers of the new car. BMW has revealed for the first time that it also plans to offer the i3 with an optional range extender (or Rex, as BMW likes to refer to it) in the form of a gasoline engine that is planned to be packaged alongside the electric motor within the rear-axle assembly.

Described as being smooth and quiet-running, the gasoline unit is used to power a generator which maintains the battery charge to ensure that the i3 can continue on electric power. Unlike the i8, the combustion engine does not provide direct drive to the rear wheels.